The present invention relates to a method and furnace for generating straightened flames in a furnace, and in particular to a method and furnace for generating straightened flames in down-fired and up-fired furnaces having multiple rows of reaction chambers (e.g., process tubes) and multiple rows of low-NOx burners.
Large down-fired and up-fired multi-row furnaces (e.g., reformer furnaces and ethylene cracking furnaces) have complex furnace gas flow patterns. Undesirable gas flow patterns can deflect flames and cause localized overheating of process tubes thereby affecting the performance of such furnaces.
A conventional steam methane reformer furnace 101 with a down-firing configuration is shown in FIG. 1. A feed gas (steam+natural gas) is supplied at the top of process tubes 180, which are filled with reforming catalyst. The feed gas is reformed in the catalyst-filled process tubes 180 to form syngas (primarily H2, CO, CH4, H2O, and CO2), which is removed near the bottom of the process tubes 180. The burners 111 generate flames 130 to provide the necessary heat for the endothermic reforming reactions. The removed syngas is then refined or purified further, for example in a pressure swing adsorber (PSA), to produce hydrogen product. The remaining gases may be sent back to the burners 111 for use as a fuel.
In FIG. 1, multiple burners 111 are illustrated with a representative furnace gas flow pattern (on the far left). The flame jet entrains furnace gases resulting in a recirculation loop in the region of the flame 130. Below the flame, furnace gases flow downward, resembling plug flow, and finally flow out through the side inlets of the refractory flue tunnels 122.
In many down-fired multi-row furnaces having conventional narrow flames, the furnace geometry allows sufficient flow area around the flame and may create a recirculation zone in the upper section of the furnace that facilitates straight flames 130, as shown in FIG. 1.
In the course of implementing new low NOx burner technology in a furnace with an aspect ratio greater than 0.8, the inventors found that substitution of low NOx burners for the conventional burners sometimes produced deflected flames that resulted in the problem of uneven heating of the reaction chambers i.e. process tubes. As depicted in FIG. 2, the new low NOx burners produced broader flames 31 which were deflected or tilted towards some of the reaction chambers 80. The flame bending and impingement on reaction chambers causes localized heating of reaction chamber walls and process gas temperature variations from row-to-row (i.e., “hot” reaction chambers in some rows, and “cooler” reaction chambers in other rows). This resulted in overheating of some reaction chamber walls, non-uniformity in wall temperatures, large deviations in process gas temperatures from one process header to another, poor radiant efficiency, and may reduce the life of the furnace and its components.
The inventors attempted some of the common methods to reduce flame deflection i.e. straighten the flames, in large box-type furnaces. These methods include:                Trimming fuel to several burners where flame impingement is occurring: By reducing the firing rate on selected burners, the flame intensity is reduced. Thus, overheating or hot-spot regions may be avoided. However, in many cases, the problem is simply moved from one furnace location and reappears at another furnace location.        Changing out fuel injection nozzles: Several burner nozzles in hot-spot regions (or burner rows) are replaced with nozzles of smaller orifice diameter (lower firing rate).        Altering flame stoichiometry: The fuel flow to hot-spot region burners is reduced but combustion air flow is kept the same. This creates fuel-lean combustion in selected areas of the furnace. Fuel-lean combustion helps in reducing peak flame temperatures and overheating.        Side row burners at lower firing rate: This is used on various large furnaces where side row burners have a firing rate that is 60% to 70% that of to the center row burners. A side row burner is a burner located adjacent a side wall of the furnace. Decreasing the firing rate of the side row burners tends to prevent these flames from bending toward the center of the furnace.        
These methods proved unsatisfactory. Further, some of these methods are not permanent solutions but rather temporary fixes which often reduce the performance of the furnace. In a furnace where furnace gas patterns are unstable, any flame stabilization or straightening efforts in one region could lead to undesirable effects in another region. In other words, the problem is simply moved from one region to another. These methods do not eliminate the problem relating to furnace gas interaction with the flames.
It is desired to have a method and a furnace for generating straightened flames using low-NOx burners in a furnace having multiple rows of burners and multiple rows of reaction chambers.
It is desired to have a method and a furnace for generating straightened down-fired or up-fired flames using low-NOx burners in a furnace having multiple rows of burners and multiple rows of reaction chambers.
It is desired to have a method and a furnace for providing uniform process gas temperatures between the multiple rows of reaction chambers, without reaction chamber overheating.
It is desired to have a method and means for reducing the deflection and resulting impingement of flames on reaction chambers in a furnace using low-NOx burners.
It is desired to have a method and a furnace for generating straightened flames in the furnace with low-NOx burners while fully utilizing the furnace combustion space for improved furnace efficiency.
It is desired to have a method and a furnace for generating straightened flames using low-NOx burners in a furnace with an aspect ratio greater than 0.8.
It is desired to have a method and a furnace for generating straightened flames using low-NOx burners in a furnace having multiple rows of burners and multiple rows of reaction chambers wherein any flame deflection is directed toward an adjacent flame rather than toward a reaction chamber.